Spark plug



Nov. 29, 1938. G, R, SMlTH 2,138,558

SPARK PLUG Filed Deo. l2, 1936 A orneys PatentredNov. 29, 1938 y y. l y y.

"UNITED STATES PATENT OFFICE George R. Smith, Flint, Mich., assignor to C. V. S. y Manufacturing Company, Flint, Mich., a corporation ol' Michigan Application December 12, 193s, serial No. 115,612

' `3 claims. (ci. 12s-169) lMy invention relates generally to spark plugs, numeral I2 for a comparatively short distance and particularly to a 'spark plug of that type below'the outstanding annulus I3 which is flat which is known in the art as a combination, twoon its underside as indicated by the numeral piece type, and an important object of my in- I4 and curved or rounded on its upper side as y 5 vention is to provide a spark plug incorporating indicated by the numeral I5. "A portion of re- 5 the features of the so-calledone-piece spark plug duced diameter I6 rises above `the annulus I3 with the advantageous features of a two-piece and on its top is the laterally extending wrench type spark plug, thereby eliminating disadvanreceiving flange AI'I which is flatk on its top vasI tageousfeatures of these types of spark plugs indicated by the numeral I8. `The lower'end of 10 while retaining and enhancing the advantageous the bore of thebushing is rounded as indicated 10 features thereof, affecting performance and by the numeral I9 forfengagement with the exmanufacture. terior of the insulator or core l as indicated ln- Another important object of my invention is Figure 1- of the drawing. to provide a combinationspark plug of the char- In the conventional one-piece spark plug conv l5 acter indicated which permits the efficient use of struction there are several advantageous features 15 a shorter insulator without impairing the rewhich are not present in theconventional twoquiredtinsulation against high tension spark piece typeof spark'plug. For instance, it rejumping between the shell and the-top of the quires a longer vand hence more expensive and center electrode, often referred to as flash-over, easilyV broken porcelain insulator to produce the so as to provide for increased emciency in the desired dielectric conditions, so that the result- 20 operation of the spark plug, while enhancing the ant spark plug is not only subject rto breakagey resistance thereof to breakage from mechanical from mechanical shock, such as a slipping wrench shock. or careless handling ordropping of the spark Other important objects of my invention will `plug,lespecially in engines in which the operatbe apparent from areading of the following deing compression isr high in the cylinders, and 25 scription taken in connection with the drawing, an unusually long path between the top exposed wherein for purposes of illustration I have shown end of thecentral electrode within the core, and a preferred embodiment of my invention. the top of the metal part of the shell or bushing In the drawingzff is required lto prevent shorting and flashing- 3o Figure 1 is a sectional elevationalrview of an over. The lengthening of this path ofthe high 30 embodiment of my invention, tension current can be easily accomplished in Figure 2 is an enlarged side elevational view a one-piecek type of spark plug, and at the same of a metallic bushing in accordance with the prestime the insulatorv may be reduced in length ent invention and showing the same with the or height in view of the fact that considerable neck and wrench receiving portion which is remetal requiredv for 'the hex bushing rin other 35 moved in the manufacture andassembling of the forms of spark plugs is eliminated. Further,- the spark plug. cost of yproviding and handling the hex bushing Referring in detail to the drawing, the numeral is eliminated in the one-piece-type of spark plug. k5 generally designates the spark plug shell which A disadvantag-eous feature of the one-piece -40 includes the usual grounded electrode or sparkspark plug device lies in the fact that in seal- 40 lng point 6,`the numeral 1 generally designates ing the metallic shell or casing, it is diliicult the porcelain or other suitable material insulato accomplish a satisfactory job Without injurtor in which the central electrode 8 is mounted ing or cracking the porcelain insulator, because and sealed, which central electrode has the sparkof the great strain necessarily placed on this ing point 9 in close proximity to the sparking part. Further it is difficult to control this pres- 45 point 6, and the binding post I0 at its upper sure so as to definitely provide for different heat end and exposed above the top of the insulator. ranges. After a one-piece spark plug has been The numeral II generally designates the all- .assembled and sealed, there can be no salvage metal bushing of the present invention which of the insulator or shell if either or both be 5 0 holds the insulator or core 'I in place in the' found defective after final testing, either in the 50 shell 5 as indicated in Figure 1 of the drawing. respect of pressure` leakage around the sealing As shown in Figure 2, the metal bushing II of the shell or electrical leakage around the is, before the finishing and assembling operacentral electrode at the top of the porcelain or l tions, composed of a cylindrical body portion pressure leakage atthis point. which is exteriorly threaded as indicated by the 'I'he kconventional two-piece type of spark plug 55 has advantages and disadvantages. 'I'he method wherebyV the metallic shell is sealed around the insulator, by engaging or fastening the threaded hex bushing to cause it to exert suilcient pressure against the top and bottom gaskets, is required to prevent loss of compression. Control of this operation is easier than the method utilized in sealing a one-piece spark plug as pointed out above, and is less likely to injure or crack the porcelain body. Furthermore, in the twopiece type of spark plug if the same be found defective in any manner after final assembly and test, the same can be disassembled and valuable parts thereof salvaged. It has been recommended that a two-piece spark plug be taken apart and cleaned after it has been in service for some time, and that it be then reassembled and restored to service. But this procedure has objectionable features which eliminates its desirability generally.

Further, the diameter and length ofthe insulator in the two-piece type of spark plug is restricted to a considerable extent because of the conventional type of hex bushing used, and this restriction results in a frangible insulator which is not able to resist any considerable mechanical shock. The vertical extension of the hex bushing above themetallic shell even after it has been tightened down, makes necessary an insulator which extends to a greater vertical height than otherwise would be required, to provide the necessary distance between the top of the central electrode and the body of the spark plug, to prevent the jumping of the high tension current therebetween. a

In the present invention a combination type is evolved which incorporates the advantageous features of the one-piece type of spark plug which produce efiicient performance and a stronger porcelain insulator, as regards mechanical shock. While in the present invention the total cost of the hex bushing is not eliminated', the cost is less than that for the one piece type of sealing, since as already pointed out, valuable parts of the onepiece spark plug must be scrapped when found defective by a test by the manufacturer.

The advantageous character of the spark plug of the present invention is made possible in main by the removal of the upper part of the hex `nut or bushing ii as shown in the drawing, thereby enabling the utilization of a shorter and hence stronger insulator which nevertheless provides the necessary insulating `distance between the top of the body of the spark plug and the upper end of the central electrode.

The reduced portion I6 and the wrench receiving flange il are cut olf by any suitable operation afterthe spark plug has been completely assembled and tested. If on final test or inspection the spark plug is found defective, it canrstill be disassembled and the valuable parts thereof salvaged, and this operation of disassembly or removal of the bushing may be accomplished in a manner to avoid injuring the spark plug in any way, and no further testing is required.

A comparison of the invention with the conventional types of two-piece spark plugs will clearly show that a much longer electrical path is made possible between the metallic shell and the top of the central electrode where a comparably long insulator is used. It follows from this that a shorter insulator 1 can be used while still providing a required distance to eliminate flash-over or shorting between the shell and the top of the center electrode. This shortening of the insulator strengthens it naturally against mechanical shock.

Another advantage of the present invention is that it cannot be disassembled or tampered with after leaving the factory. Further, the invention is of great importance because of the `lmaller type of spark plugs being manufactured in in creasing quantities, such as the 14 mm., and its value is relatively greater in the still smaller sizes, such as the 10 mm. and the 12 mm.

Although I have shown and described herein a preferred embodiment of my invention, it is to be definitely understood that I do not desire to limit the application of the invention thereto, and any change or changes may be made in the materials, and in the structure and arrangement of the parts, within the spirit of the nvention and the scope of the subjoined claims.

What is claimed is:-

l. A method of making a spark plug having an increased dielectric distance between the sealing bushing and the upper end of the central electrode exposed at the top of the core, said method comprising removing the wrench receiving portion from the top of the bushing and reducing the top of the bushing so as to be substantially flush with the top of the shell of the spark plug.

2. A spark plug comprising a tubular shell having an interior shoulder located intermediate the upper and lower ends thereof, a tubular bushing threaded into the upper end of said shell and having a lateral projection engaged with the upper end of the shell and substantially flush therewith, an insulator surrounded by said shell and said bushing, said insulator having a laterally projecting annular part resting on said interior shoulder with the lower end of said bushing engaging the top of said annular part.

3. A spark plug comprising a tubular shell having an interior shoulder, an insulator surrounded by said shell and having an annular projection resting directly upon said interior shoulder whereby the upper part of the insulator extends above the upper end of the shell to a conventional height, a tubular bushing surrounding said insulator andvthreaded into the upper end of said shell, said bushing engaging its lower end directly with the top of said annular projection, and an annulus on the upper end of said bushing engaging an upper part of said shell, the top of said annulus being rounded to merge substantially flush with the top of said shell.

GEORGE R. SMITH. 

